1. Field of the Invention
The present invention relates in general to a dielectric ceramic composition, particularly to a dielectric ceramic composition suitable for forming dielectric resonators, for example, which are used with microwave (high-frequency) devices.
2. Discussion of the Prior Art
Keeping pace with recent developments of various microwave devices such as pocket or portable telephones, there has been a growing demand for small-sized high-performance dielectric resonators used in such devices. In particular, a dielectric ceramic used to form such dielectric resonators is required to have the following physical and structural features;
(1) The dielectric ceramic should have a sufficiently high specific dielectric constant (.epsilon.r), to reduce the size of a resonator when the resonator is operated at a given frequency.
(2) The dielectric ceramic should have a sufficiently high Q value, to reduce the transmission loss of the resonator formed of the dielectric ceramic.
(3) The temperature coefficient (.tau.f) of the resonance frequency of the resonator formed of the dielectric ceramic should be sufficiently low, to minimize changes of the operating characteristics of the resonator with respect to temperature.
There have been known various dielectric ceramic compositions used for forming such dielectric resonators. For example, laid-open Publication No. 58-20905 of examined Japanese Patent Application discloses a dielectric ceramic composition whose major components consist of BaO and TiO.sub.2, a portion of which may be substituted by another element or other elements. A dielectric ceramic formed of the disclosed composition has a relatively low dielectric constant of around 30 .about. 40, and a relatively low Q value. Another example of dielectric ceramic composition is disclosed in laid-open Publication No. 59-23048 of examined Japanese Patent Application, which has a composite perovskite structure such as Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3. The dielectric ceramic formed of this composition has a relatively high Q value, but suffers from a relatively low dielectric constant of around 30 .about. 40.
There is also known a dielectric ceramic composition as disclosed in laid-open Publication 56-102003 of unexamined Japanese Patent Application, whose major components consist of BaO, TiO.sub.2, Nd.sub.2 O.sub.3 and Bi.sub.2 O.sub.3. While this dielectric ceramic composition has a relatively high dielectric constant, the temperature coefficient of the resonance frequency of the resonator formed of this ceramic composition is undesirably high, making it difficult for the resonator to provide satisfactory operating characteristics. A further example of dielectric ceramic composition as disclosed in laid-open Publication No. 57-21010 of unexamined Japanese Patent Application, whose major components consist of BaO, TiO.sub.2, Nd.sub.2 O.sub.3 and Sm.sub.2 O.sub.3, has a relatively high temperature coefficient of the resonance frequency, which causes some problems in practical use. Still another example of dielectric ceramic composition is disclosed in laid-open Publication No. 1-227303 of unexamined Japanese Patent Application, which contains BaO, TiO.sub.2, Nd.sub.2 O.sub.3 and Al.sub.2 O.sub.3 as major components. The dielectric ceramic formed of this composition has a relatively low Q value resulting in an increase in the transmission loss of the obtained resonator, owing to the presence of Al.sub.2 O.sub.3 in the composition.